Stability Guidance issued Booklet on: version date 2002-10-29 FOR MASTERS AND WATCH KEEPERS for Powered Vessels
Contents 1 Information specific to this vessel 1 2 General cautions 3 3 Stability check lists 3 4 Explanatory notes 5 5 Some definitions 10 6 Additional guidance for Masters 11 Reduced righting moment This booklet was prepared by a working group comprising representatives from: The Royal Institution of Naval Architects The Royal Yachting Association The Royal National Lifeboat Institution The Jubilee Sailing Trust The Multihull Offshore Cruising and Racing Association The Wolfson Unit for Marine Technology and Industrial Aerodynamics in consultation with a wide range of experienced seafarers. Illustrations by Sarah Selman Caution It should be noted that, while every care has been taken in the preparation of this booklet, the advice and information given cannot take account of every exceptional circumstance. Final responsibility for the safety of the vessel rests with the Master. Owners may find it helpful to display pages 1 to 4 in a prominent place for the information of the crew.
Section 1 Information specific to the power vessel This page to be completed for the specific individual vessel Information specific to the power vessel: Type of vessel (design/class and length) Identification/registration number 1 Approved limit of operation according to stability assessment 1 Angle of first deck-edge immersion Angle of first immersion of coaming/bulwark (eg: Smith-Jones 440 13.4m) (eg: 2 = up to 60nm from a safe haven) Computed 2 (eg: 29º) (eg: 34º) Master s estimate 3 Angle of first downflooding (hatch and ventilators open) Angle of vanishing stability (AVS) (eg: 42º) (eg: 70º) Openings labelled as required to be secured closed when at sea (eg: skylights, emergency escape hatches, foredeck hatch) This vessel is designed to float after sustaining localised damage to the hull or fittings, excluding bulkheads. 4 This vessel will continue to float when fully loaded, even if fully flooded or swamped. 4 1 Where this exists. 2 Data to be supplied or verified by the Certifying Authority. 3 Data to be provided by the master, especially when computed data is not available. These figures are not required to be checked by the Certifying Authority. Where both are provided, the Master s estimate should not exceed the computed value. 4 Text to be deleted unless the appropriate requirements are fulfilled. 1
Section 1 Information specific to the power vessel This page to be completed for the specific individual vessel Master s standing orders How to heave-to 5 Certifying Authority approving the data (computed and below) Stability assessed assuming Maximum permitted number of persons Maximum permissible weight to be carried (kg) Minimum operating displacement (tonnes) 6 Maximum loaded displacement (tonnes) 6 Outfit items included in the assessment 7 Liferaft(s) (eg: 2 by 8 person liferafts in containers on coachroof) Mast(s) Dinghy Radar (eg: alloy extrusions, fore = 7.4kg/m, aft = 2.9kg/m) (eg: rigid dinghy in stern davits) (eg: radar antenna weighing 4kg on wheelhouse roof) Other topweight 2 5 To be completed by the Master. Not required to be checked by the Certifying Authority. 6 Where this is available. 7 All items likely to affect the vertical centre-of-gravity significantly should be included.
Section 2 General cautions General cautions Final responsibility for the safety of the vessel rests with the Master. see note Adding weight high up or moving weight higher in the vessel 1 (including lifting from any high point) reduces the stability. Excessive list or trim adversely affects the stability and handling 2 of the vessel. Breaking waves are capable of inverting most smaller vessels, 3 and should therefore be avoided if possible. Section 3 Stability check lists Stability check lists Before putting to sea see note Remove bilge water and check that bilge suctions are clear. 4 Check that freeing ports and deck drains are clear. 5 Ensure that openings labelled as required to be closed when 6 at sea are secured shut. Locate all seacocks and close those not required to be open. 7 Thoroughly secure all loose gear, on deck and below. 8 Ensure that the vessel is not overloaded, if applicable by checking the freeboard mark is visible. Check air tanks or flotation elements to ensure that they are 9 effective. See explanatory notes for more information. 3
Section 3 Stability check lists Stability check lists At sea in normal conditions see note Breaking waves higher than the beam of your vessel, if taken 10 beam-on, may cause capsize. If resonant rolling develops, alter heading and/or speed. 11 Tight turns at speed may lead to capsize. 12 Where possible avoid shallow water near to where fast ferries 13 are operating. Be aware of the risk of burying the bow at speed in following 14 or quartering seas. Before the onset of severe weather, be sure you know the 15 Master s instructions for heaving-to. At sea in rough conditions Close all downflooding openings not essential for the working 16 of the vessel. Keep weatherdeck hatchways and doors closed whenever 17 possible. Take particular care to avoid areas where severe breaking waves 18 are likely to occur as they can cause capsize. Actively steer the vessel to avoid the most hazardous waves. 19 In following seas, be aware of the risk of broaching and 20 pitchpoling. Emergency conditions In reduced visibility, or after a collision, or if any compartment 21 is being flooded, call the Master and close all watertight doors. See explanatory notes for more information. 4
Section 4 Explanatory notes Explanatory notes General Cautions 1 Weight added above the centre-of-gravity of the vessel, or taken off below the centre-of-gravity reduces the stability. What may seem to be a small effect near the upright is greatly increased at 90º of heel. The ability to recover from a knockdown may be drastically reduced by a seemingly small increase in centre-of-gravity height. The effect of a suspended weight is as if it were located at the point of Righting moment suspension. A vessel can be capsized even in perfectly calm water by lifting an excessive weight, or by raising the point of suspension too high. A very slow rolling motion is a sign that this condition may be being reached. Righting moment Capsizing moment Special care should therefore be taken when using a lifting device. 2 A list or permanent heel in one direction reduces the margins of stability in that direction. A trim down by the bow may reduce the directional stability, increasing the tendency to turn and increasing the possibility of broaching in following seas, or of shipping water over the bow. A trim down by the stern may increase the directional stability, thus reducing the ability to manoeuvre, and increasing the possibility of shipping seas over the stern. 3 All monohull vessels under about 24m length are capable of being inverted by a breaking wave of sufficient size. To be dangerous in this respect, a breaking wave must have a height exceeding the beam of the vessel. 5
Section 4 Explanatory notes Before putting to sea 4 Bilge water, if present in any quantity, reduces the effective stability of the vessel. As the vessel heels, loose water moves to the lower side, thus increasing the initial heel angle ( freesurface effect ). Many vessels make some bilge water in rough conditions, so regular bilge checks at sea are advisable. Pump suctions are often prone to clogging with debris that has found its way to the bilges, so suction points should be checked and cleared before sailing. Reduced righting moment 5 Cockpits or decks with bulwarks rely on drains and freeing ports to enable any water shipped to drain away quickly. It is important that such fittings are working properly, because trapped water reduces the stability in two ways: Firstly water trapped on deck has the same free-surface effect as loose bilge water (see note 4 above). Secondly water trapped high in the vessel raises the centre-of-gravity (see note 1), and its weight also reduces freeboard so that more water is likely to be shipped. Deck drains and freeing ports (especially those fitted with non-return flaps) must therefore be in proper working order. 6 Most vessels are fitted with various types of opening that may admit water if left open when at sea, namely: portlights, skylights, engine or deck hatches. The stability is assessed assuming that all openings marked to be kept shut at sea have been closed. If such openings are not closed before putting to sea, a progressive accumulation of bilge water is likely with consequent adverse effect on the stability see note 4 above. 7 Some seacocks must be left open for the proper working of the vessel s systems, eg: engine cooling, drains and scuppers, fire pump suction. Some may cause inadvertent flooding if left open. Typically these include those relating to toilets and sinks fitted relatively low in the vessel and which may become submerged when the vessel is heeled to large angles. 8 Loose gear, if not properly secured, will fall to the low side of the vessel when it heels. Apart from the risk of injury or damage, this has a similar effect on stability as loose water see note 4. It is especially important that heavy items (such as batteries or spare anchors) are very well secured against movement, even at very large angles of heel, for example after being heeled to 90º or more. 6
Section 4 Explanatory notes 9 If the vessel is fitted with air tanks or flotation spaces, survival after swamping or damage may be severely impaired if such spaces are not well maintained and regularly checked for water. Drain plugs on all such spaces should therefore be opened at regular intervals to ensure that leakage has not occurred. At sea in normal conditions 10 The bigger the wave relative to your vessel, the greater the risk of being rolled by a beam sea. This risk can be significantly reduced by not taking such waves beam-on. See also note 18. Rigid inflatable boats are not generally subject to as much risk. 11 Resonant rolling occurs when an initial disturbance causes a rolling motion that progressively grows due to the action of the wind or waves. Vessels may experience resonant rolling if encountering a series of fairly regular beam waves. The waves do not have to be especially large, but may just have a period similar to the natural rolling period of the boat. A change of heading and/or speed reduces resonant rolling motions. 12 Tight turns at speed generate substantial centrifugal forces that can increase the risk of capsize. 13 A fast ferry develops a different kind of wash from a conventional ship, wash that may only reach your vessel some 15 to 20 minutes after it has passed. Such wash comprises a few very short and steep waves that are normally not dangerous. However, when such waves encounter relatively shallow water they can become dangerously high and steep, sufficient to swamp smaller boats. It is therefore wise to avoid shallow water in the vicinity of routes where such ferries operate at high speed. 14 All vessels, if driven too hard in following or quartering seas, can bury their bows into the sea. In extreme cases this may lead to pitchpoling due to the bows digging into the trough of the wave while the stern is being lifted by wind and sea. The stern is then lifted OVER the bows! The solution is to reduce speed, and perhaps deploy a drogue. 7
Section 4 Explanatory notes 8 15 To be prepared for the eventuality, make sure that you know the best technique for heaving-to for your specific vessel. This may require some experiment in suitably fresh conditions. The Master s instructions are given at the front of this booklet. At sea in rough conditions 16 In rough weather, small amounts of water regularly finding their way into the vessel over a lengthy period can accumulate alarmingly. One litre every ten seconds becomes 1080 litres or over a tonne every three hours! Before the going gets rough, all potential downflooding openings should be closed unless they really need to be open. On smaller vessels the only ventilators that should be left open are those fitted with water traps. Special care should be taken with any hatches or ventilators fitted well off the centreline of the vessel, as these will be the first to become immersed when the vessel heels. 17 The main hatchway opening should be kept as small as practicably possible by keeping the sliding top closed, and one or more washboards in position. If hatches or doors need to be opened in a rough sea, close and secure them as soon as possible afterwards. 18 The crest of a breaking wave contains a massive amount of energy that can exert a powerful heeling effect on any vessel, especially those under about 24m length. Such waves are one of the most likely causes of capsize, so that it is prudent to avoid areas where they are likely to occur. Breaking waves are especially likely when wind is against tide, when the wind is rapidly increasing in strength, or after a sudden wind shift when waves are coming from different directions. The most dangerous breaking waves are likely to occur where: there are tide races or overfalls marked on the chart the wind is contrary to the current direction the sea bed shoals rapidly, even if the least depth seems to be very generous near lee shores, especially those that shoal steeply in areas such as headlands where the current is strongest
Section 4 Explanatory notes 19 It is often possible, by alert helmsmanship, to steer the vessel away from threatening waves, but this is a tiring task and requires frequent changes of helmsman. However, avoiding such waves is much more important than keeping a steady course. 20 Broaching in following seas occurs when a wave crest picks up the stern, causing the bow to dig in and the boat to slew rapidly through 90º. In large waves, the violence of this uncontrolled manoeuvre can result in the boat being thrown onto its side, sometimes being completely inverted. If the tendency to broach is persistent, consult the Master and slow the vessel down, in extreme cases by towing long bights of heavy warps or a strong drogue. Emergency conditions 21 If any compartment is being flooded, whether through holing, failure of a fitting or downflooding, any watertight doors should be closed immediately, only being opened temporarily for access. Similar action should be taken as a precaution when navigating in reduced visibility, or crossing shipping lanes, as the risk of collision damage is greatest. Some vessels do not slow down sufficiently when visibility is poor, so that a collision may occur within seconds of the other vessel being sighted. 9
Section 5 Some definitions Some definitions Angle of Vanishing Stability (AVS) Area of Operation Broaching The angle of heel at which, in calm water, a vessel continues to an inversion rather than returning to the upright. One of seven categories assigned under the MCA Small Commercial Vessel and Pilot Boat Code, based on distance from refuge. A violent turning and heeling effect created in following or quartering seas. Buoyancy Capsize Design Category Downflooding Flooding Flotation Inversion Knockdown Pitchpole Righting Moment Swamping The upward force produced when a vessel is immersed in water. When a vessel is heeled to any angle from which it cannot recover without assistance. One of four categories defined in the EU Recreational Craft Directive based on sea and wind conditions. Flooding through openings that are normally above the calm water level. When a vessel fills with water relatively slowly, eg: through submerged downflooding openings, or through leaks of fittings below the waterline. Means of providing buoyancy in a vessel after swamping or flooding, eg: air tanks, air bags or foam material. When a vessel becomes completely upside down in the water. When a vessel is heeled to about 90º. When the vessel inverts end-for-end, eg: stern over bow. The moment tending to return a vessel to the upright, being the product of vessel weight and righting lever. When a vessel is suddenly filled with water from above, eg: by waves. 10
Section 6 Additional guidance for masters Additional guidance for masters Stability check lists Explanatory notes 11
Section 6 Additional guidance for masters Stability check lists General cautions Be aware of the implied weather limitations of the Area of 22 Operation assigned to your vessel. In rough conditions Heave to if the vessel is labouring. If heaving-to ceases to be practical, the usual options are to: lay to a sea anchor, or run before the waves towing a drogue or warps. Emergency conditions see note After a collision with another vessel, do not disengage until 23 any need to evacuate has been established. If aground on a falling tide, ensure that the vessel lists away from deep water by moving onboard weights. If aground on rock or a sharp obstruction, check the extent of damage before attempting to free your vessel, as it may sink quickly. Even if partially swamped many craft will stay afloat and should not automatically be abandoned. See explanatory notes for more information. 12
Section 6 Additional guidance for masters Explanatory notes 22 Areas of operation are defined in the MCA Small Commercial Vessel and Pilot Boat Code in terms of geographical limitations. These designations contain implicit assumptions regarding the severity of the conditions that may be experienced. A vessel assigned to Area 0 is not restricted and therefore may be assumed to at times experience storms (force 10) with accompanying very high sea states, significant wave heights exceeding five metres. A vessel assigned to Area 1 may experience severe gale (force 9) winds and associated sea states, significant wave heights exceeding four metres. A vessel assigned to Area 2 may experience gale (force 8) winds and associated sea states, but is expected to seek sheltered waters before severe gale force conditions are met. Significant wave heights up to four metres may be encountered. A vessel assigned to Area 3 may experience near-gale (force 7) winds and associated sea states, but is expected to seek sheltered waters before gale conditions are met. Significant wave heights up to three metres may be encountered. Vessels assigned to Areas 4 to 6 may experience force 6 winds and associated sea states, but are expected to seek sheltered waters before force 6 conditions are exceeded. Significant wave heights up to two metres may be encountered. Emergency conditions 23 In the event of a collision, if locked together the two vessels should not be separated immediately. This is for two reasons: Firstly, the withdrawal of one vessel will increase the flow of water into the other, potentially resulting in rapid sinking. A properly designed vessel will usually survive bow damage without immediate difficulty. Secondly, evacuation of the more badly damaged vessel is much easier if the two remain in contact with one another. Therefore carefully assess the condition of both vessels before attempting to separate them. 13
Stability Guidance Booklet for Masters and Watch Keepers for Powered Vessels The Maritime and Coastguard Agency Spring Place, 105 Commercial Road Southampton SO15 1EG Tel: 023 8032 9100 Fax: 023 8032 9447 MCA 133 MCA February 2005 Email: infoline@mcga.gov.uk 24 hour infoline: 0870 600 6505 www.mcga.gov.uk